1. Field of the Invention
The present invention relates generally to magnetic sensing elements for use in hard disk devices and magnetic sensors. In particular, it relates to a magnetic sensing element having excellent read characteristics that can adequately control the magnetization of free magnetic layers even with narrower tracks and to a method for fabricating the same.
2. Description of the Related Art
FIG. 36 is a partial cross-sectional view of a conventional magnetic sensing element viewed from the face that opposes a recording medium. Hereinafter, this face is referred to as the “opposing face”.
Referring to FIG. 36, a composite film 108 is formed on a substrate 101. The composite film 108 is constituted from an antiferromagnetic layer 102, a pinned magnetic layer 103, a nonmagnetic material layer 104, and a free magnetic layer 105. A hard bias layer 106 is disposed at each side of the composite film 108. An electrode layer 107 is formed on each hard bias layer 106.
The magnetization of the pinned magnetic layer 103 is pinned in the Y direction in the drawing by an exchange coupling magnetic field generated between the pinned magnetic layer 103 and the antiferromagnetic layer 102. The magnetization of the free magnetic layer 105 is oriented in the X direction in the drawing by a longitudinal bias magnetic field.
As shown in FIG. 36, the track width Tw is determined by the width of the free magnetic layer 105 in the track width direction (the X direction). The track width Tw is becoming ever smaller as recording densities become higher.
However, the magnetic sensing element having the structure shown in FIG. 36 cannot properly control the magnetization direction of the free magnetic layer 105 when the track width is small.
First, according to the structure shown in FIG. 36, the width of the free magnetic layer 105 must be decreased to read narrower tracks. As the track becomes narrower, regions in the free magnetic layer 105 affected by strong longitudinal bias magnetic fields from the hard bias layers 106 occupy large portions of the free magnetic layer 105. The regions affected by the strong longitudinal bias magnetic fields then form dead regions that do not readily respond to external magnetic fields. Since the dead regions relatively expand, the sensitivity of the magnetic sensing element is degraded as the tracks become narrower.
Secondly, the hard bias layer 106 may easily become magnetically discontinuous from the free magnetic layer 5. This problem is particularly acute when a bias underlayer composed of Cr is provided between the hard bias layer 106 and the free magnetic layer 105.
Such magnetic discontinuity intensifies the adverse effects of demagnetizing fields at the two ends of the free magnetic layer 105 in the track width direction, often resulting in a magnetization disturbance in the free magnetic layer 105, i.e., a buckling phenomenon. The buckling phenomenon occurs more frequently over large areas in the free magnetic layer 105 as the track becomes narrower. This results in instability in the read waveform.
Thirdly, as the gap becomes narrower, part of the longitudinal bias magnetic fields from the hard bias layers 106 escapes to shield layers (not shown) disposed above and below the magnetic sensing element shown in FIG. 36. This disturbs the magnetization state of the shield layers and weakens the longitudinal bias magnetic field supplied to the free magnetic layer 105. Thus, the magnetization of the free magnetic layer 105 cannot be properly controlled.
Recently, in order to overcome these problems, exchange bias methods are beginning to be employed to control the magnetization of the free magnetic layer 105. One exchange bias method provides an antiferromagnetic layer disposed on a free magnetic layer.
A magnetic sensing element of an exchange bias type is manufactured, for example, through the steps shown in FIGS. 37 and 38. FIGS. 37 and 38 are partial cross-sectional views of the magnetic sensing element viewed from the opposing face.
In the step shown in FIG. 37, an antimagnetic layer 2 composed of a PtMn alloy is formed on a substrate 1. Then a pinned magnetic layer 3 composed of a magnetic material, a nonmagnetic material layer 4, and a free magnetic layer 5 composed of a magnetic material are deposited on the antimagnetic layer 2. A Ta film 9 for preventing oxidation of the free magnetic layer 5 when exposed to air is formed on the free magnetic layer 5.
As shown in FIG. 37, a lift-off resist layer 10 is then formed on the Ta film 9. Part of the Ta film 9 not covered with the resist layer 10 is completely removed by ion milling. At this time, part of the free magnetic layer 5 under the Ta film 9 is also removed. The removed portion is indicated by broken lines in the drawing.
Next, in the step shown in FIG. 38, a ferromagnetic layer 11, a second antiferromagnetic layer 12 composed of an IrMn alloy, and an electrode layer 13 are sequentially formed on each of the exposed portions of the free magnetic layer 5 at the two sides of the resist layer 10. The liftoff resist layer 10 is removed at the end to complete the exchange bias magnetic sensing element.
In the magnetic sensing element shown in FIG. 38, the track width Tw is determined by the gap between the ferromagnetic layers 11 in the track width direction (the X direction in the drawing). The magnetization directions of the ferromagnetic layers 11 are firmly pinned in the X direction in the drawing by exchange coupling magnetic fields generated between the ferromagnetic layers 11 and the second antiferromagnetic layers 12. As a result, two side portions A of the free magnetic layer 5 located under the ferromagnetic layers 11 are strongly magnetized in the X direction by ferromagnetic coupling with the ferromagnetic layers 11. On the other hand, the central portion B of the free magnetic layer 5 in the track width Tw region is only weakly magnetized to be in a single magnetic domain state such that the magnetization of the central portion B can rotate in response to external magnetic fields.
The exchange bias magnetic sensing element manufactured through the steps shown in FIGS. 37 and 38, however, has the following problems.
First, during the ion milling in the step shown in FIG. 37, not only the Ta film 9 but also part of the free magnetic layer 5 is removed. Moreover, inert gas, such as Ar, used during the ion milling readily enters the free magnetic layer 5. This damage in the free magnetic layer 5 destroys the crystal structure in surface portions 5a of the free magnetic layer 5 and causes lattice defects (a so-called mixing effect). As a result, the magnetic characteristics of the surface portions 5a of the free magnetic layer 5 are often degraded.
Ideally, only the Ta film 9 is removed during ion milling in the step shown in FIG. 37 without removing the free magnetic layer 5. However, in practice, it is difficult to control the milling operation in such a manner because of the thickness of the Ta film 9. The Ta film 9 is formed to have a thickness of approximately 30 to 50 Å. Such a large thickness is required to properly prevent the oxidation of the free magnetic layer 5.
When the Ta film 9 is exposed to air or field-annealed to generate exchange coupling magnetic fields between the pinned magnetic layer 3 and the ferromagnetic layer 11 and between the antimagnetic layer 2 and the second antiferromagnetic layer 12, the oxidized portion expands, and the entire thickness of the Ta film 9 becomes larger than that immediately after deposition. For example, a Ta film 9 having a thickness of approximately 30 Å immediately after deposition may expand to approximately 45 Å in thickness by the oxidation.
In order to effectively mill the Ta film 9 expanded by the oxidation, high-energy is required. Since high-energy ion milling has a high milling rate, it is almost impossible to stop milling at the moment the Ta film 9 is completely removed. In other words, when the energy is high, the margin of position for stopping the milling must be set large. Accordingly, part of the free magnetic layer 5 under the Ta film 9 is removed, and significant damage is inflicted on the free magnetic layer 5 by high-energy ion milling, resulting in degradation of the magnetic characteristics.
Secondly, it is difficult to stop ion milling partway of the free magnetic layer 5 shown in FIG. 37 because the free magnetic layer 5 is formed to have a thickness of 30 to 40 Å and is milled using high energy. In the worst case, the two side portions A of the free magnetic layer 5 may be completely removed by ion milling. As described above, because the thickness of the free magnetic layer 5 is small, it is difficult to stop ion milling partway of the free magnetic layer 5.
Thirdly, the surface of the free magnetic layer 5 exposed to the ion milling exhibits degraded magnetic characteristics due to the damage inflicted by the milling. Thus, the magnetic coupling (ferromagnetic exchange interaction) between the free magnetic layer 5 and the ferromagnetic layers 11 is insufficient. As a result, the thickness of the ferromagnetic layers 11 must be increased.
However, when the thickness of the ferromagnetic layers 11 is increased, the exchange coupling magnetic fields with the second antiferromagnetic layers 12 become weak. As a result, the magnetization of the two side portions A of the free magnetic layer 5 cannot be firmly pinned. This causes a problem of side reading. The resulting magnetic sensing element cannot properly meet the demand for narrower tracks.
Moreover, when the thickness of the ferromagnetic layers 11 is excessively large, static magnetic fields from ends of the ferromagnetic layers 11 may readily reach the central portion B of the free magnetic layer 5, thereby degrading the sensitivity of the central portion B, which has a rotatable magnetization in response to external magnetic fields.
As described above, it has been impossible to manufacture a magnetic sensing element that can meet the demand for narrower tracks through the above-described steps of milling the two side portions of the Ta film 9 to expose the free magnetic layer 5 and depositing the ferromagnetic layers 11 and the second antiferromagnetic layers 12 on the exposed portions of the free magnetic layer 5. This is because the magnetization of the free magnetic layer 5 cannot be properly controlled in this structure.